1. Field of the Invention
The present invention relates to a three-dimensional shape measurement method and a three-dimensional shape measurement apparatus for acquiring height information for a sample and a color omnifocal image thereof.
2. Description of the Related Art
A microscope interferometer measurement method is generally known as a method of measuring the three-dimensional shape of a micro-sample with high precision. For example, in a microscope interferometer measurement method disclosed in the specification of U.S. Pat. No. 5,133,601, light having a wide wavelength band emitted from a white light source (e.g., a halogen lamp or mercury lamp) is applied to a sample by an interference objective lens. In this microscope interferometer measurement method, an interference figure is imaged in which light (hereinafter, measurement light) reflected from the sample interferes with light (hereinafter, reference light) reflected from a reference surface provided in the interference objective lens, thereby acquiring an interference image.
The interference objective lens includes a Michelson-type interference objective lens, a Mirau-type interference objective lens, etc. In both the Michelson-type interference objective lens and the Mirau-type interference objective lens, reference optical paths are formed by beam splitters provided in these interference objective lenses. In this reference optical path, a reference surface is provided at a position conjugate with an object side focal position of the interference objective lens.
The light source emits white light, that is, light having a short coherence length. Thus, the intensity of the interference is maximized when an optical path difference between the measurement light and the reference light is zero. The reference surface is disposed at a position conjugate with the object side focal position of the interference objective lens. Thus, the optical path difference becomes zero, that is, the interference intensity is maximized when the sample is in focus. The interference intensity decreases as the optical path difference increases along with a change in the relative distance between the interference objective lens and the sample. The region which permits the measurement of the change of interference intensity generally has a significantly small optical path difference of several micrometers or less, and this nature is used to measure the height (height information) of the sample.
That is, an image pickup unit (hereinafter, an image pickup device) sequentially acquires interference images of the sample every time a drive unit causes the interference objective lens to scan (move) in an optical axis direction (hereinafter, a Z-direction). A control unit obtains a position in the optical axis direction at which the interference intensity is maximized for all the interference images picked up by the image pickup device. This permits the three-dimensional shape of the sample to be obtained.
Here, a luminance transition curve (an interference pattern in the form of waveforms (interference waveforms)) acquired by the control unit when the interference objective lens scans in the Z-direction is generally referred to as an interferogram. This interferogram is shown in FIG. 22.
The envelope (a broken line shown in FIG. 22) of the interferogram is determined by the coherence length of the light source. Periodic changes within the envelope are attributed to the interference of light, and their period is about λ/2 where λ is the centroid wavelength of the light source.
Methods of obtaining a position where the interference intensity is maximized from this interferogram include, for example, the method using a low pass filter disclosed in the specification of U.S. Pat. No. 5,133,601, and a method using Hilbert transformation disclosed in Applied Optics/Vol. 31, No. 14/10 May 1992 “Three-dimensional image realization in interference microscopy”.
However, conventional apparatuses using the methods mentioned above are limited to the acquisition of the height information alone, and are not capable of obtaining information on the color of the sample, the difference of reflectance, etc. For example, when samples have the same height but are formed of different compositions, the conventional apparatuses using the methods described above are not capable of recognizing the difference of compositions from information on the measured heights because they can not obtain the information on the color of the sample, the difference of reflectance, etc.
Furthermore, the sample has ultramicroscopic irregularities when the sample has micro-flaws or when dust, etc., in the atmosphere sticks to the sample. If the image pickup device is not capable of sufficient spatial sampling of the ultramicroscopic irregularities, information on the height of ultramicroscopic parts is not detected and is overlooked.
Moreover, when attention is focused on one interference image used in the height measurement, the reference light is superposed on an imaging area in the interference image. Therefore, the interference image looks like an image having uniform flare light. Thus, the interference image is an observation image having lower contrast than that of an image obtained by a general optical microscope, and is not sufficient for the quality of a luminance image.